As a general method of forming a wellbore, e.g. for extraction of oil or gas from a formation, a drilling operation typically involves mounting a drill bit on a drilling assembly (the “bottom hole assembly”) at the lower end of a drill string and rotating the drill bit against the bottom of a hole to penetrate the formation, thereby creating a wellbore.
A drilling fluid, such as a “drilling mud”, typically circulates down through the drill string, passes via the drill bit, and returns back to the surface, usually in the annular portion between the drill string and the wall of the wellbore. The drilling fluid serves a number of purposes, including lubricating the drill bit and cooling the drilling assembly. However, the drilling fluid can also be suitably pressurized to provide sufficient hydrostatic pressure at the wellbore wall to prevent the flow of fluids into the wellbore from the surrounding formation.
Such relatively high pressure can produce undesirable mechanical forces on the formation, which may lead to wellbore damage. In particular, as the wellbore deepens the hydrostatic pressure at the lower end of the wellbore can be significantly higher than the pressure near the entrance aperture of the wellbore.
Accordingly, in the past, it has been necessary periodically to halt the drilling operation and to provide a casing within the wellbore to provide structural support, the casing typically being cemented in place to the wall of the wellbore. U.S. Pat. Nos. 4,760,882, 4,547,298 and 4,768,593 describe such a process in more detail.
In particular, in the method disclosed in these patents a cement mixture is located in the annular region between the casing and the wall of the wellbore and is set (solidified) in situ by exposure to gamma-radiation produced by e.g. a Co-60 source lowered down the encased wellbore on a probe.
However, to be able to drill a deep wellbore or a wellbore in a relatively unstable formation (e.g. a shale or clay formation), the drilling operation must be halted repeatedly to allow the wellbore to be so encased. This has disadvantages in that it delays the extraction of valuable oil and/or gas from the well and consequently has a negative economic impact.
GB Patent No. 2 363810 recognizes this disadvantage and discloses a method of lining a wellbore during the drilling operation. The method allows a material to form a layer supported by a wall of the wellbore, where the material is selected so that the shear modulus of the layer is smaller than the shear modulus of the formation forming the wall of the wellbore. When the wellbore is sufficiently deep, a casing can be cemented in place in the wellbore in the usual way.
GB Patent No. 2420572 discloses a process of forming a layer on the wall of a wellbore when drilling through a shale and/or clay formation. The interaction of water with a shale and/or clay wellbore wall can cause the shale and/or clay to swell, thereby contracting the wellbore and possibly leading to softening and disintegration of the wall of the wellbore. The lining disclosed in GB Patent No. 2420572 is intended to prevent absorption of water into the shale and/or clay, rather than to provide the wellbore wall with mechanical support.
The process of GB2420572 includes the steps of bringing a drilling fluid which includes a graft polymer into contact with the clay or shale wall of the wellbore and letting the graft polymer form a layer on the wall. The graft polymer is a copolymer, formed by the reaction of an oligomeric or polymeric substrate with at least one ethylenically unsaturated monomer. The reaction to form the graft polymer, e.g. performed in a reaction chamber, is conducted in the presence of a type II photo initiator and by the action of actinic radiation. The resulting graft polymer being suitable for adding to a drilling mud for use as described above.
However, the use of such a process has its disadvantages. The extent of mechanical support offered by a layer of graft polymer is likely to be small. The graft polymer is relatively indiscriminate, and it is therefore likely to be difficult to control accurately the thickness or location of the layer which it forms on the wall of the wellbore.